Monolithic multiple solar cells

ABSTRACT

A monolithic multiple solar cell includes at least three partial cells, with a semiconductor mirror placed between two partial cells. The aim of the invention is to improve the radiation stability of said solar cell. For this purpose, the semiconductor mirror has a high degree of reflection in at least one part of a spectral absorption area of the partial cell which is arranged above the semiconductor mirror and a high degree of transmission within the spectral absorption range of the partial cell arranged below the semiconductor mirror.

This nonprovisional application is a continuation of U.S. applicationSer. No. 14/988,250, which was filed on Jan. 5, 2016, which is acontinuation of U.S. Application No. 14/203,669, which was filed on Mar.11, 2014, which is a divisional of U.S. application Ser. No. 11/813,217,which was filed on Jul. 2, 2007, which is a National Stage applicationof PCT/EP2005/014022, and was filed on Dec. 23, 2005, and which claimspriority to German Patent Application No. DE 10 2005 000 767.8, whichwas filed in Germany on Jan. 4, 2005, and which are all hereinincorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a monolithic multiple solar cell made fromelements of the 3rd and 5th main groups of the periodic table ofelements with an integrated semiconductor mirror.

Description of the Background Art

A corresponding double solar cell (also called tandem cell) is knownfrom the publication “Prog. Photovol: Res. Appl. 2001; 9:165-178, F.Dimroth et al. Metamorphic Ga.sub.yIn.sub.1-yP/Ga.sub.1-xIn.sub.xAsTandem Solar Cells for Space and for Terrestrial ConcentratorApplications at C>1000 Suns.” As the integrated semiconductor mirror aBragg mirror is used. The Bragg mirror is thus epitaxially applied on aGaAs substrate, on which in turn the double solar cell GaInP/GaInAs,which has an n to p polarity, is epitaxially applied.

From Physica E. 2002, Vol. 14, p. 84-90, M. Yamaguchi: “Multi-junctionSolar Cells and Novel Structures for Solar Cell Applications”, simpleand multiple solar cells are known, wherein a Bragg reflector may bedisposed on the rear side of the simple solar cells.

From the Journal of Electronic Materials, Vol. 21, No. 3, 1992, S. M.Vernon et al. “Growth and Characterization of Al.sub.xGa.sub.1-xAs Braggreflectors by LP-MOCVD”, a GaAs solar cell with a Bragg reflector on asilicon substrate is known.

From the publication “Solar Energy Materials & Solar Cells 68 (2001), p.105-122, M. Z. Shvarts et al. “Radiation Resistant AlGaAs/GaAsConcentrator Solar Cells with Internal Bragg Reflector”, a GaAs simplesolar cell is known, under which a Bragg mirror is disposed. The solarcell used is a cell with a p to n polarity.

From WO-A-2004/017425, a multiple solar cell is known, in which backreflectors, which carry out the function of the electric mirrors inrelation to the minority carriers, run between the individual solarcells.

Lattice-matched triple solar cells ofGa.sub.0.5In.sub.0.5P/Ga.sub.0.99In.sub.0.01As/Ge are market leadersamong the space solar cells. The best cells reach 30-31% efficiencyunder the space solar spectrum AMO. In production, so far a maximumaverage efficiency of at least 28.3% was achieved. A decisive factor isthe radiation hardness of the cells. According to a typical irradiationwith 1.times.10.sup.15 cm.sup.-2 1 MeV electrons, the efficiency ofthese cells is degraded to 24.3%. This corresponds to a residual factorof 86%.

Quintuple to sextuple solar cells are described in “Solar CellManufacturers Come Back Down to Earth” by M. Hatcher,Compoundseminconductor.net magazine, November 2003.

SUMMARY OF THE INVENTION

It is an object of the invention to further develop a monolithicmultiple solar cell of the kind described above in order to improve theradiation stability in such a way that the end-of-life (EOL) efficiencyis increased. At the same time, current generation of the individualpartial cells should not be substantially affected. The expenditureduring production of the solar cell should not be noticeably influenced.

The object is attained according to the invention in that the multiplesolar cell comprises at least three partial cells, the semiconductormirror is disposed between two partial cells and has several layers withrefractive indices that at least differ from each other and thesemiconductor mirror has a high degree of reflection in at least onepart of the absorption range of the partial cell or partial cellsdisposed above the semiconductor mirror and a high transmission degreefor radiation in the spectral absorption range of the partial cell orpartial cells disposed underneath the semiconductor mirror.

As the reflection degree R is identified herein the wavelength-dependentreflection averaged across a specific spectral range. Ideally, thisspectral range comprises the entire spectral absorption range of thepartial cell located over the semiconductor mirror. In the application,frequently a smaller spectral range is sufficient, in particular areflection band close to the band gap energy of the partial cell locatedabove it with a half-width value HWB of nm 40 mm HWB≦300 nm.

A high degree of reflection R means therein that the average reflectionover the reflection band amounts to R≧0.3, preferably R≧0.7, andespecially 0.7≦R≦1.

The wavelength-dependent transmission is identified herein as a degreeof the transmission T averaged across the spectral absorption range ofthe partial cell or partial cells located under the semiconductormirror. A high degree of transmission T means therein that T≧0.5,preferably T≧0.85, and especially 0.9≦T≦1.

An optical mirror in the form of a monolithically integratedsemiconductor mirror for multiple solar cells consisting of orcomprising a compound semiconductor of groups III and V of the periodictable is proposed according to the invention, wherein the multiple solarcell comprises three or more partial cells. The semiconductor mirror isinstalled under the nth solar cell of the multiple solar cell by meansof a suitable epitaxy process. Suitable epitaxy processes are, forexample, MOVPE (metal organic vapor phase epitaxy) or MBE (molecularbeam epitaxy) or VPE (vapor phase epitaxy), just to name a few.

The semiconductor is made of a plurality of thin semiconductor layerswith different refraction indices. The number, order, and thickness ofthe layers are selected in such a way that part of the solar spectrum isreflected back into the nth partial cell of the m partial solar cells,which comprises multiple solar cells. Therein, m>n as well as m=3. Thethickness of the nth partial cell is cut in half by using a mirror suchas this, without drastically reducing the absorption in the partialcell. At the same time, the layers are selected in such a way that thesemiconductor mirror has high transmission in another part of the solarspectrum. In this way, it is ensured that the current generation in the(n+1)th partial cell, which is to say the cell that runs underneath thesemiconductor mirror, as well as the other partial cells that arelocated underneath the semiconductor mirror, is not reduced beyond acritical degree.

The integration of a corresponding semiconductor mirror is especiallyadvantageous if one of the partial cells of the multiple solar cell ismade of a material with a low diffusion length or when the diffusionlength in one of the partial cells substantially deteriorates during theapplication. The thickness of the nth partial cell can be cut in half byusing the semiconductor mirror without strongly reducing the absorptionin this partial cell. The combination of the introduction of asemiconductor mirror and a simultaneous reduction of the cell thicknesshas a positive effect on current generation. A more advantageousgeneration profile over the depth of the active layers of the partialcell is achieved. It is particularly significant that the averagedistance of the generated minority carriers to the depletion layer isstrongly reduced due to the lesser cell thickness. This leads to anincreased probability that the minority carriers will encounter thedepletion layer during diffusion and will thus contribute to the currentgeneration. The recombination is reduced in this way.

An important application example is theGa.sub.0.5In.sub.0.5P/Ga.sub.0.99In.sub.0.01As/Ge triple solar cell,which has become one of the most widely used cell in satellites inrecent years. In space, the solar cells are exposed to a spectrum ofhighly energetic particles, for example, electrons and protons withenergies in the MeV range. This radiation can lead to a degradation ofthe solar cell, especially to increased recombination, and therewith toa shorter diffusion length in the semiconductor material. While theGaInP upper cell and the Ge lower cell are relatively insensitive tospatial radiation, the Ga.sub.0.99In.sub.0.01As central cell exhibitsstrong degradation. In typical radiation doses, such as approximately1.times.10.sup.15 cm.sup.-2 1 MeV electrons, the diffusion length inGa.sub.0.99In.sub.0.01As decreases so strongly that only a part of thegenerated minority carriers reaches the depletion layer. A strongdegradation of the generated current or the quantum efficiency of thecentral cell is the consequence. The degradation of the central cellthus determines the degradation of the triple solar cell. Thisdegradation can be clearly reduced by inserting the semiconductor mirrorbetween the central and lower cells and by simultaneously cutting inhalf the thickness of the central cell according to the invention. Thelayers of the semiconductor mirror are selected in this case in such away that high reflection within the spectral range of higher energiesthan those of the band gap of Ga.sub.0.99In.sub.0.01As of approx. 1.4 eVis achieved. At the same time, sufficient transmission of lower energiesmust be achieved, so that sufficient current is generated in the Gelower cell. In the case of theGa.sub.0.5In.sub.0.5P/Ga.sub.0/99In.sub.0.01As/Ge triple solar cell, theGe lower cell generates an excess of current (approx. 30 mA/cm.sup.2 incomparison with the upper partial cells with approx. 17 mA/cm.sup.2),whereby the requirements of transparency of the semiconductor mirror forlower energies are not quite so high.

While in the case of theGa.sub.0.5In.sub.0.5P/Ga.sub.0.99In.sub.0.01As/Ge triple solar cell allthe partial cells are bred with the same lattice constant, which islattice-matched to the substrate, in the current development presentlytriple solar cells are studied, comprising lattice-mismatched layers.

Typical material combinations are GaInP/GaInAs/Ge or alsoAlGaInP/GaInAs/Ge, wherein the indium content in one or in both of theupper partial cells is higher than in the previously mentioned triplesolar cell. The application of the semiconductor mirror according to theinvention is similar for these cell concepts. The requirements oftransparency of the semiconductor mirror for lower energies is somewhathigher herein than that of the energy band gap of the utilized GaInAs,since the excess of generation in the Ge lower cell is less.

The semiconductor mirror can also be similarly used for lattice-matchedand lattice-mismatched quintuple solar cells, which are likewisecurrently under development for spatial applications.AlGaInP/GaInP/AlGaInAs/Ge is, for example, a typical materialcombination for such a cell.

While in the previously discussed cell concepts the problem of the lowdiffusion length was caused by the application in space and theradiation existing therein, there are also concepts for multiple solarcells that comprise materials with inherently lower diffusion length.Ga.sub.0.5In.sub.0.5P/Ga.sub.0.99In.sub.0.01As/Ge triple solar cells canbe expanded by adding a further partial cell of a material with anenergy band gap of approx. 1 eV intoGa.sub.0.5In.sub.0.5P/Ga.sub.0.99In.sub.0.01As/1 eV material/Gequadruple solar cells. The GaInNAs material can be used, for example, toimplement an energy band gap of 1 eV with simultaneous lattice matchingto the Ge substrate. However, this material has exhibited until now verylow diffusion lengths, for which reason it has not been possible toachieve better results with quadruple solar cells than with triple solarcells.

The integration of a semiconductor mirror according to the inventionunder the GaInNAs partial cell with a simultaneous adaptation of thecell thickness can, in this case, likewise be successfully applied. Thelayers of the semiconductor mirror are selected therein in such a waythat a high reflection of energies that are greater than 1 eV andsimultaneously a high transmission of energies that are lower than 1 eVare achieved. The requirements of transparency of the semiconductormirror for lower energies are herein very high, since the Ge lower celldoes not have an excess of generation. A quadruple solar cell such asthis is not only suitable for use in space, but also for applications interrestrial concentrator systems.

Sextuple solar cells for spatial applications are also known. These areobtained, for example, from the expansion of anAlGaInP/GaInP/AlGaInAs/GaInAs/Ge quintuple solar cell with a furtherpartial cell of a material with an energy band gap of approx. 1.0-1.1eV. A possible material combination is thusAlGaInP/GaInP/AlGaInAs/GaInAs/GaInNAs/Ge. The integration of one or eventwo semiconductor mirrors is carried out according to the teaching ofthe invention.

Examples of multiple solar cells can also be found in the publication“Proceedings of the 19.sup.th European Photovoltaic Energy Conference”,7-11 Jun. 2004, Paris, p. 3581-3586, M. Muesel et al: “European Roadmapfor the Development of III-V Multi-Junction Space Solar Cells.”

If, as is customary for the epitaxy of a multiple solar cell structure,a Ge substrate is used, which is activated during the epitaxy and formsthe Ge lower cell, the teaching of the invention can also be applied tosolar cells in which the epitaxy is carried out on a silicon or Si/Gesubstrate.

Independently from this, a further development of the invention providesthat the layer of the semiconductor mirror that precedes the subsequentpartial cell has a lattice structure, which is adapted to that of thepartial cell.

The layers of the semiconductor mirror are made of a compoundsemiconductor of the 3rd and 5th main groups of the periodic table orcomprise these. The compound semiconductors can be doped herein with Si,Te, Zn, C, Mg and/or Se.

The layers can be made of a compound semiconductor material, such asAlGaInAs material and/or AlGaInP material or can comprise these, whereinAlGaInAs includes GaAs, InAs, AlAs, GaInAs, AlGaAs, AlinAs and/orAlGaInP includes GaP, InP, AlP, GaInP or AlInP.

The respective layer of the semiconductor mirror should have a thicknessd with 2 nm≦d≦300 nm, especially 10 nm≦d≦150 nm.

The semiconductor mirror comprises n layers, wherein 10≦n≦50, especially15≦n≦35. The total thickness D of the semiconductor mirror should amountto 500 nm≦D≦4000 nm, especially 750 nm≦D≦2500 nm.

The invention teaches very generally that, with n layers of thesemiconductor mirror, at least two layers have different refractionindices and/or at least two layers have different material compositionsand/or at least two layers have different thicknesses. Three or morelayers can, of course, also have different material compositions ordifferent indices of refraction or different thicknesses.

Due to the plurality of different material layers, especiallyadvantageous properties are achieved with reference to the reflectionand transmission, as well as the possibility of passivation of theboundary layer to the partial cell located above it.

The invention provides especially that the semiconductor mirror isintegrated between the central and lower cells of a GaInP/GaInAs/Getriple solar cell, wherein preferably the GaInAs central cell has athickness of d.sub.m with 500≦d.sub.m≦2500 nm, and especially 800nm≦d.sub.m≦2000 nm.

The invention provides furthermore that one or more semiconductormirrors are integrated in a GaInP/GaInAs/GaInNAs/Ge quadruple solarcell, wherein a semiconductor mirror is disposed especially between theGaInNAs and the Ge partial cells.

An integration of the semiconductor mirror is also possible in aquintuple solar cell, which is made, for example, of the partial cellsAlGaInP/GaInP/AlGaInAs/GaInAs/Ge, wherein the semiconductor mirrorshould be disposed between the GaInAs and the Ge partial cells.

In the event that a sextuple solar cell is used, in which one or moresemiconductor mirrors are integrated, the solar cell should comprise thepartial cells AlGaInP/GaInP/AlGaInAs/GaInAs/GaInNAs/Ge.

Particularly good EOL efficiency can be produced when the semiconductormirror has a high half-width value. An optimal half-width value (HWB)would be the one that includes the entire absorption range of thepartial cell located above it. In this way, however, limitations of thedegree of reflection would have to be tolerated or a high materialconsumption would be required. In order to achieve optimization, thehalf-width value should be between 50 nm and 300 nm, especially between80 nm and 150 nm.

The layers of the semiconductor mirror should also be designed in such away that the degree of reflection R amounts to R≧0.3, especially R≧0.7,and preferably to 0.7≦R≦1 and the degree of transmission T amounts toT≧0.5, especially T≧0.85, and preferably 0.9≦T≦1.

A special case of the semiconductor mirror according to the invention isthe Bragg reflector. The latter consists usually of a periodic overlaylattice made of 2 materials with different indices of diffraction. Thethickness of the respective layers of material is constant. In this way,reflections up to almost 100% can be achieved, depending on the numberof overlay lattice periods. The Bragg mirrors are used, for example, insurface emitting lasers.

The semiconductor mirror according to the invention, which has thefunction of an optical mirror, represents very generally an extremelygood reflector with reference to the partial cells disposed above it anda long pass filter with reference to the partial cell disposedunderneath the semiconductor mirror.

The teaching of the invention can be advantageously used especially ifone of the partial cells is a multiple solar cell made from a materialwith a low diffusion length or if the diffusion length considerablydeteriorates during use in one of the partial cells. The thickness ofthe nth partial cell can be cut in half by using the semiconductormirror without substantially reducing the absorption in the partialcell. The combination of the integration of a semiconductor mirror andthe simultaneous reduction of the cell thickness has a positive effecton current generation. On the one hand, a more favorable regenerationprofile is achieved over the depth of the active layers of the partialcell and, on the other hand, the lesser cell thickness causes a strongreduction of the average distance of the generated minority carriers tothe depletion layer. This leads to an increased probability that theminority carriers will encounter the depletion layer during thediffusion and will thus contribute to current generation. Therecombination is consequently reduced. As a consequence of this, anincrease of the current generated in the partial cell is achieved, whichis limited by a low diffusion length based on the teaching of theinvention. Especially the radiation stability of theGa.sub.0.99In.sub.0.01As central cell of aGa.sub.0.5In.sub.0.5P/Ga.sub.0.99In.sub.0.01As/Ge triple solar cell canbe clearly improved.

The overall thickness of the multiple solar cell can be further reducedby means of the integration of a semiconductor mirror. Correspondingimprovements of the radiation stability result in the case of theGa.sub.0.99In.sub.0.01As or lattice-mismatched GaInAs partial cells inmultiple solar cells with three or more partial cells. Through theintegration of a semiconductor mirror, the teaching of the invention canprovide a decisive contribution to the successful application of newmaterials with low diffusion lengths, such as GaInNAs as partial cell ina multiple solar cell.

The semiconductor mirror is characterized by suitable materials, whereina lattice match with the partial cells disposed on the semiconductormirror is carried out. At least two materials with a high difference inthe indices of refraction are used in order to achieve high reflection.Materials with an equally great or greater band gap than that of thepartial cell located above it are used in order to prevent theoccurrence of an absorption for smaller energies. The thickness of thepartial cell applied on the semiconductor mirror can be reduced,especially cut in half, by means of the integration of the semiconductormirror.

In order to configure the semiconductor mirror, conventional epitaxyprocesses are used, which allow the deposition of numerous layers ofdifferent materials. As an example should be mentioned MOVPE, MBE, VPE,and the like. The semiconductor mirror is also deposited during theepitaxy of the solar cell structure.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitiveof the present invention, and wherein:

FIG. 1 shows a schematic configuration of a multiple solar cell withintegrated semiconductor mirror,

FIG. 2 shows a reflection of an ideal semiconductor mirror, and

FIG. 3 shows a simulated reflection of a semiconductor mirror.

DETAILED DESCRIPTION

The schematic configuration of a multiple solar cell 10 with asemiconductor mirror integrated according to the invention can be seenin FIG. 1. The solar cell 10 comprises m partial cells 12, 14, 16, 18,which have been epitaxially applied on a substrate 20. Between the (n)thcell 16 with m>n and the (n+1)th cell 18, a semiconductor mirror 22 isintegrated, which has also been deposited during the epitaxy of thesolar cell structure. Suitable epitaxy processes to be considered arethose that are suitable for the deposition of numerous layers ofdifferent materials. MOVPE, MBE (molecular beam epitaxy), or VPE (vaporphase epitaxy) can be mentioned as examples, without causing as aconsequence a limitation of the teaching of the invention.

The multiple solar cell 10 is especially a triple solar cell, whereinthe upper cell is made of Ga.sub.0.5In.sub.0.5P, the central cell ismade of Ga.sub.0.99In.sub.0.01As, and the lower cell is made of Ge. Thesemiconductor mirror 22, which comprises several layers, is integratedin particular between the lower cell made of Ge and the central cellmade of Ga.sub.0.99In.sub.0.01As. The layered structure is such that atleast two layers of different materials, different thicknesses, anddifferent indices of refraction are provided.

Through the selection of the materials, layer thicknesses, and indicesof refraction, in the ideal case a reflection behavior as that shown inFIG. 2 is achieved. Maximum reflection is thus obtained for energiesgreater than the band gap energy of the nth partial cell, that is,maximum reflection is achieved in the exemplary embodiment of the triplecell with a Ga.sub.0.99In.sub.0.01As central cell having a band gapenergy of >1.4 eV or 880 nm. For energies that are lower than the bandgap energy of the nth cell, the reflection is minimal or thetransmission is maximal. Transmission losses through absorption in thesemiconductor mirror can be prevented or kept negligibly small byselecting suitable materials, for example, GaAs and AlGaAs.

The uppermost layer of the semiconductor mirror 22 can be made of GaInPand at the same time be the rear side field for theGa.sub.0.99In.sub.0.01As central cell located above. GaInP is used asmaterial, since it has very good properties as rear side passivation.The remaining layers of the semiconductor mirror 22 in the exemplaryembodiment are made of three different materials:Ga.sub.0.99In.sub.0.01As, Al.sub.0.2Ga.sub.0.8As, andAl.sub.0.8Ga.sub.0.2As. There is an essential difference with respect tothe Bragg reflector, which is made of only two different materials.Furthermore, various layer thicknesses are also in the example, while inthe classic Bragg reflector all the layers of one material have the samethickness.

A further essential characteristic of the layer sequence of thesemiconductor mirror 22 is that it reaches, on the one hand, a highreflection for energies above the band edge of the partial cell locatedabove it, but has, on the other hand, also a low reflection or hightransmission for lower energies. The reflection of the semiconductormirror on the boundary surface to the Ga.sub.0.99In.sub.0.01As partialcell disposed above is shown in FIG. 3 to illustrate this. A highreflection is achieved for wavelengths between 800 nm and 900 nm. Forwavelengths greater than 900 nm, however, the reflection is low. This isan important property of the semiconductor mirror, which ensures thatthe current generation in the Ge lower cell is not substantially reducedby the semiconductor mirror 22.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

What is claimed is:
 1. A solar cell comprising: a first cell formed of aplurality of first layers, at least two of the first layers have adifferent doping from one another; a second cell formed of a pluralityof second layers, at least two of the second layers have a differentdoping from one another, a single tunnel diode, the first cell and thesecond cell being connected in series by the single tunnel diode; and aBragg reflector formed of a plurality of third layers, the third layerspartially reflecting incident light back to the first cell and allowinga portion of the incident light to pass to the second cell, the Braggreflector being arranged between and adjacent to the first cell and thesecond cell.
 2. The solar cell according to claim 1, wherein the solarcell is a monolithic solar cell.
 3. The solar cell according to claim 1,wherein the plurality of third layers contain at least 10 layers.
 4. Thesolar cell according to claim 1, wherein the plurality of third layershave at least two layers that have a different thickness or a differentrefraction index or are made of different materials.
 5. The solar cellaccording to claim 1, wherein the Bragg reflector is directly adjacentto the first cell.
 6. The solar cell according to claim 1, wherein theBragg reflector is directly adjacent to the second cell.
 7. The solarcell according to claim 1, wherein the Bragg reflector has materialswith a band gap energy that is equal with or greater than the firstcell.
 8. The solar cell according to claim 1, wherein the solar cell isa single solar cell stack.
 9. The solar cell according to claim 1,wherein no additional cell is formed between the first cell, secondcell, and the Bragg reflector.